dataclasses.FrozenInstanceError via .delegates.txt_parser.common

[khaeru]

I just ran into a cryptic:

dataclasses.FrozenInstanceError: cannot assign to field '__traceback__'

…using pint 0.21 and dask 2023.4.0 together. The exception is actually thrown from within contextlib:

  File "/home/khaeru/vc/genno/genno/core/computer.py", line 499, in get
    result = dask_get(dsk, key)
             ^^^^^^^^^^^^^^^^^^
   File "/home/khaeru/.venv/3.11/lib/python3.11/site-packages/dask/local.py", line 557, in get_sync
    return get_async(
           ^^^^^^^^^^
   File "/home/khaeru/.venv/3.11/lib/python3.11/site-packages/dask/local.py", line 425, in get_async
    with local_callbacks(callbacks) as callbacks:
   File "/usr/lib/python3.11/contextlib.py", line 189, in __exit__
    exc.__traceback__ = traceback
    ^^^^^^^^^^^^^^^^^
   File "<string>", line 4, in __setattr__

I see that 08b9807 set @dataclass(frozen=False) in several places in pint.errors with the commit message "Using frozen=False in errors to avoid problems with dask". Upon some digging, I discovered the offending class in my case was actually this one:

pint/pint/delegates/txt_defparser/common.py

Lines 21 to 22 in dd89fb8

	@dataclass(frozen=True)
	class DefinitionSyntaxError(errors.DefinitionSyntaxError, fp.ParsingError):

This appears to have been missed in the earlier commit.

[khaeru]

Perhaps worth noting that this is filed as a bug in CPython per se: python/cpython#99856

Depending on the resolution, maybe pint could eventually go back to frozen=True if that's desired.

[hgrecco]

I like frozen dataclasses. It makes things easier to reason around. But it seems that exceptions cannot be frozen in dask. I was wonder what would happen if we add a traceback get/set property and use this to change its value.